I saw a “news” story asking the question, “Could Earth defend itself from an ET invasion?”
Overlooking the fact that there would be no realistic reason for the aliens to invade because everything they would want could be found elsewhere in the Solar System, except, of course for our “Earth-like” environment and the life found here, the answer to the question is, “NO!”
I remember one of the science fiction writers, I think Jerry Pournelle or Larry Niven, but it might have been Harlan Ellison, say, “All they would have to do is stand back and throw rocks at us.”
What that meant, simply, was that an alien race that wanted to invade could soften us up by dropping asteroids on us. Think of all the documentaries that have aired in the last ten years that tell us about “asteroid apocalypse.” An asteroid some 300 feet in diameter (or something about (100 meters) would do incredible damage. Move to something about a half mile, and continents disappear, the global climate would be shot for centuries, and billions would perish.
Go much larger than that and you have an extinction level event. Pushing asteroids around wouldn’t be all that difficult for a spacefaring race. And rather than pushing a dinosaur killer (yes, I know that the dinosaurs probably weren’t extinguished by an asteroid, but the term is great) into an orbit that would collide with Earth, they could push hundreds of smaller ones into that orbit. It could turn out to be carpet bombing the world with no danger to themselves.
The point is that given our current technology, we simply couldn’t reach out to touch them. If they are inside of the orbit of the moon, then we could shoot at them, but our chemical rockets would be moving slowly enough that they could intercept and destroy them regardless of how many we launched and right now there aren’t all that many when talking interstellar war.
Or, they could just move out of the way.
Or, throw rocks at the missiles.
So, if the aliens wished to invade, given our current technology, they would win. If they decided they needed to occupy Earth, they certainly could do that as well, but why expose themselves to our retaliation. Why not just sit there in orbit and dictate their terms to us? If we chose to fight back, throw another rock.
Strategically speaking, they have no motivation to engage in a fight especially one on the planet’s surface. Independence Day never dealt with that question. “V” in its various incarnations did, to a degree… the “visitors” wanted to engage in some sort of trade or communication with a hidden agenda. Damon Knight’s story, “To Serve Man,” (which also a Twilight Zone episode) dealt with the reason for a landing rather than an invasion. But, at least there was a reason for the contact.
But without any additional reason, they have no motivation to land or to invade. If we launched something into orbit to attack them, they could withdraw to the orbit of, say, Saturn, and it would take us years to get there. They could evade us simply. And then attack at their leisure.
In the end, the answer to the question is that he who controls the high ground wins the battle. We can’t actually fight a battle in space, which means the aliens have the high ground. Or… as was learned in the Iraq war, Iraqi tanks could engage at one mile. The problem? The Abrams tanks could engage at two miles. In other words, they could stand out of range of the Iraqis and destroy their armor. Such is the situation we would face with the aliens in orbit. We couldn’t successfully engage and we would lose.